1. Field of the Invention
The present invention relates to a photonic fibre triggered laser device.
2. Description of the Related Art
The production principle of triggered high intensity and short duration laser pulses has been known for a long time. It consists in preventing the regenerative amplification of a wave in a cavity including a laser medium by introducing losses greater than the gain of the laser medium. After a pumping period which enables to store a significant energy in the gain medium, the optical transmission of the trigger is suddenly increased so as to enable the creation of an intra-cavity wave which is amplified very rapidly and gives rise to the emission of a light pulse. The duration of the pulses produced is conversely proportional to the gain of the laser medium, and proportional to the length of the laser cavity. Conventionally, the gain medium is a laser bar and the trigger may be an acousto-optical or electro-optical modulator. Besides, the possibility of generating very good quality and high average power light beams by using double sheath optical fibres the core is doped with an ion exhibiting a laser transition is known. These systems exhibit a relatively small active zone (typically less than 10 microns in diameter) and operate hence mainly continuously so as to prevent the faces of the fibre from being damaged by high energy laser pulses (damage threshold approx. 20 to 50 J/cm2 for 10 ns-pulses).
Laser fibres with photonic layers or MPF (for multiclad photonic fibre) are also known and have been presented in the article by J. Limpert, N. Deguil-Robin, I. Manek-Hönninger, F. Salin, F. Röser, A. Liem, T. Schreiber, S. Nolte, H. Zellmer, A. Tünnermann, J. Broeng, A. Petersson, and C. Jakobsen, “High-power rod-type photonic crystal fiber laser,” Opt. Express 13, 1055-1058 (2005). MPF lasers include optical amplifiers with glass fibre formed of a doped core and of at least one peripheral sheath ensuring the guiding of a produced wave. The core is doped by a rare earth ion, neodymium or ytterbium generally. The guiding is ensured by the implementation of a photonic structure obtained by a geometrical assembly of channels or aerial capillaries (holes). This structure lowers artificially the index encountered by the wave produced and enables mono-mode propagations for fibre core diameters of the order of 50 μm. This large core diameter enables to spread the energy of the wave produced over a greater surface and to push back both fundamental limitations of fibre amplifiers, i.e. flow handling and non-linear effects. With such a technology it may be contemplated to generate relatively short laser pulses from 1 ns to 30 ns with energies of the order of 1 mJ to 10 mJ.
These MPF lasers may exhibit extremely high gains thanks to the very high confinement of the gain zone. Such confinement imposes conventionally a great absorption length and a high limitation in the energy produced by triggering. However it is very difficult to maintain losses greater than the gain during the pumping period. We have still been able to show that particular photonic fibres could be used for simultaneously diminishing the absorption length and increasing the size of the active zone. Limpert et al (Conference on Advanced Solid State Photonics, Vienna, February 2005) have used one of these fibres so as to generate high energy nanosecond pulses. Nevertheless, they resorted to a very rapid triggering (<5 ns) using a Pockels cell so as to block the cavity during the pumping phase. The laser is then limited to rates of the order of 100 kHz and the triggering system is particularly costly. Moreover the system is sensitive to the polarisation of the wave propagating in the resonator and its efficiency may be diminished by the de-polarisation during the propagation through the fibre.